This invention relates to a magnetic recording media of the type comprising a base having a coating of magnetic particles in a polymeric binder, and, more particularly, to an improved binder for such coatings.
Magnetic recording media of the kind herein contemplated, which will be described as being a magnetic tape, comprises a flexible base or tape having a surface coated with magnetic particles in a suitable binder. Such magnetic tape, when used in electronic data processing machines, for example, must withstand extreme mechanical abuse in that it is moved at high speeds with the coating in physical contact with stationary metal parts, such as tape guides and transducer heads. Due to this physical contact with the stationary metal parts, many of the presently used tape coatings are rapidly abraded, with some of the abraded material depositing in the equipment, thereby adversely affecting its operation. Many applications further require that the tape be capable of withstanding relatively high temperatures as, for example, in aircraft crash recorders and in satellite recorders for space exploration. In some applications, too, it is necessary that the magnetic surface be periodically cleaned, usually with an organic solvent, thus imposing the additional requirement that the binder have good solvent resistance. Moreover, the binder must be securely anchored to the base material, typically formed of polyethylene terephthalate or cellulose acetate, and the coating must have good flexibility so as to adhere to the backing material even when crimped or otherwise deformed or subjected to dimensional changes caused by changes in temperature, or by stretching.
A variety of binders have been and are used in the manufacture of magnetic tapes, the selection being made primarily on the basis of their resistance to abrasion. In the late 1950's, for example, highly flexible thermoplastic materials were used as the binder for the magnetic particles; however, the resulting films had poor wear resistance and were somewhat spatially unstable at higher ambient temperatures. Among the binders subsequently used are flexible thermosetting expoxy-based resins, an example being an expoxy-ureaformaldehyde resin as described in U.S. Pat. No. 3,148,082. The resin is combined with a curing agent selected from the class consisting of polyamide resins or triethylene tetramine.
Another known binder material, described in U.S. Pat. No. 3,149,995, consists of a polymeric matrix material consisting essentially of at least one solid, soluble, thermoplastic copolymer of vinyl alcohol and at least one member of the group consisting of vinyl esters, vinyl acetals, vinyl halides, or vinylidene halides, and a diisocyanate-based elastomer. A variation of this coating material is disclosed and claimed in U.S. Pat. No. 3,150,995 which consists of an elastomer binder selected from a particular class of cross-linked diisocyanate-based compositions.
Another conventionally used binder consists of partially hydrolyzed copolymers of vinyl chloride and vinyl acetate blended with a soft elastomer, such as diisocyanate-based elastomers, in proper proportions to give the desired adhesion and mechanical properties.